Plastic propeller fans applied to cooling fans for automobile radiators and air-conditioning condensers are installed in narrow engine compartments and are required to be lightweight and are thus under the constraint that the size in the airflow direction (depth dimension) cannot be increased. Therefore, the depth dimensions of blades, a hub, a shroud, etc. that constitute a propeller fan are made as thin as possible. In addition, the gap sizes between them and the radiator, the condenser, or the like should be as small as possible.
Such minimization of the gap sizes causes the cooling fans installed in severe operating environments influenced by vibrations, high temperature, water pressure due to water splashed during driving on a flooded road or the like to interfere with the radiator or the condenser due to deformation of the blades. In particular, thin plastic propeller fans rotated at high speed are easily deformed due to their shape, which could cause a serious accident due to interference with the core of the radiator or the condenser. Known solutions for preventing deformation of plastic propeller fans include, in addition to increasing the strength of the blades by increasing the wall thickness thereof, configurations in which the outermost peripheries of the blades are joined together with a ring member, as shown in Patent Literatures 1 and 2.
{Citation List}
{Patent Literature}
    {PTL 1} Japanese Unexamined Patent Application, Publication No. Hei 8-159090    {PTL 2} Japanese Unexamined Patent Application, Publication No. 2005-106003